School me on wiring

1. Similarly to what everyone already said - a multimeter and a test light will be all you need in 90% of the time. With a multimeter you can test if two wires have continuity, if a wire has power, if a wire is grounded, the current through a circuit - learn how to use one.
2. An understanding of Ohms law will help with some of the less "on/off"-y things (Fuel sending units, ballast resistors, temp sensors)
3. Learn the electrical components (relays, solenoids, ignition coils if you're feeling ambitious)